What do car diagnostics actually reveal?
A modern car has many control units linked in a network: engine, gearbox, brakes, suspension, air conditioning, comfort functions. Each one monitors its sensors and stores anomalies as fault codes, often with the conditions under which they occurred. Electronic diagnostics means interrogating those units with diagnostic tools suited to each make.
Reading the codes is only the starting point. A code that names a sensor does not always mean the sensor is at fault: a corroded connector, a chafed wire or low battery voltage produce the same symptoms. That is why we never replace a part on the strength of a code alone.
- reading fault codes from every control unit, and clearing them only after analysis;
- live data: temperatures, pressures, engine speed, voltages, sensor values, observed with the engine running and on the road;
- resets and adaptations after servicing or a part replacement, within what the tools allow for each car;
- checking the power supply to the control units and the vehicle network.
How does electrical fault finding work?
We work in order. First, we listen to what you have noticed and when: from cold, in the rain, after a long drive. Then we reproduce the fault if possible, read the memories, and verify by measurement — voltage, continuity, resistance, voltage drop under load. The car’s wiring diagram guides the search; measurement confirms it.
Once the cause is found, you receive a quote itemised line by line. No repair is started without your approval, and the detailed invoice records what was done: useful for the car’s history and for its eventual sale.
Why does the battery go flat on its own?
When parked, a modern car still draws a little current: alarm, memories, control units on standby. When a module fails to go to sleep, a relay sticks or an accessory has been badly wired, this parasitic drain empties the battery within days.
To trace it, we measure the current drawn with the car locked, once the control units have gone to sleep, then isolate the circuits one by one. On many modern cars, a new battery must also be registered with the charging management system: an often forgotten step that otherwise upsets charging. For a car that is seldom driven, storage with the battery kept on charge avoids many surprises.
Classic car electrics: looms, earths and charging
On a car from the 1950s to the 1980s, the electrics are simpler, but they have aged. Loom insulation hardens and cracks, connectors corrode, earths deteriorate with rust. Many apparently mysterious faults on a classic — dim headlamps, an erratic gauge, hesitant starting — come down to a poor earth or a tired contact.
During mechanical repair and overhaul, the electrics are dealt with alongside everything else: a rebuilt engine deserves faultless ignition and charging.
- wiring looms: inspection, local repair or renewal, with wire and sleeving true to the original in appearance if you wish;
- charging: dynamo or alternator, regulator, belt, charging voltage measured;
- starting: starter motor, solenoid, cables and earth straps;
- ignition: points and condenser, coil, distributor, leads and plugs, or electronic ignition depending on the car;
- relays, fuses, switches and stalks.
Engine warning light on: what should you do?
A steady amber engine warning light signals a stored fault, most often related to emissions or a sensor: the car can usually still be driven, but the fault should be read without delay. A flashing engine light, or a red oil pressure or temperature warning, means you should stop.
Brake and ABS warnings deserve particular attention. A dirty wheel-speed sensor or a damaged harness is enough to trigger them, but they can also indicate low fluid or worn pads. The diagnosis then continues on the braking system itself.
Diagnostics tailored to each make
Every manufacturer has its own logic, known weak points and procedures. For a Porsche, from the 911 Carrera 3.2 to the 992, our Porsche diagnostics page covers the points specific to the marque, from the DME relay of classic 911s to the over-rev report.
Diagnostics are often part of regular servicing: reading the memories at every service reveals an emerging fault before it leaves the car stranded. As an independent workshop, we tell you before starting what is possible on your car.